Butadiene extraction pre-absorber

ABSTRACT

A process for recovering butadiene from a C 4  fraction is disclosed. The process may include: contacting a mixed C 4  stream comprising butane, butene, and butadiene, with a solvent comprising an organic solvent and water in a butadiene pre-absorber column to recover an overheads fraction comprising at least a portion of the butane, butene, and water, and a first bottoms fraction comprising the organic solvent, butadiene, and at least a portion of the butene; and feeding the first bottoms fraction to a butadiene extraction unit to recover a butene fraction, a crude butadiene fraction, and a solvent fraction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application, pursuant to 35 U.S.C. §119(e), claims priority to U.S.Provisional Application Ser. No. 61/703,409, filed Sep. 20, 2012, whichis herein incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

Embodiments disclosed here relate to recovery of butadiene from a mixedhydrocarbon stream. More specifically, embodiments disclosed hereinrelate to recovery of a crude butadiene stream from a mixed C4hydrocarbon stream using a butadiene pre-absorber column which may beintegrated with an extractive distillation column for the efficientrecovery of butadiene.

BACKGROUND

Butadiene is an important base chemical and is used, for example, toprepare synthetic rubbers (butadiene homopolymers,styrene-butadiene-rubber or nitrile rubber) or for preparingthermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers).Butadiene is also converted to sulfolane, chloroprene and1,4-hexamethylenediamine (via 1,4-dichlorobutene and adiponitrile).Dimerization of butadiene also allows vinylcyclohexene to be generated,which can be dehydrogenated to form styrene.

Butadiene can be prepared from saturated hydrocarbons by refiningprocess or by thermal cracking (steam cracking) processes, in which casenaphtha is typically used as the raw material. In the course of refiningor steam cracking of naphtha, a mixture of methane, ethane, ethene,acetylene, propane, propene, propyne, allene, butenes, butadiene,butynes, methylallene, C₄ and higher hydrocarbons are obtained.

Typical processes to recover butadiene from mixed C₄ streams includeextractive distillation processes, which may incorporate use ofselective solvents. Examples of extractive distillation processes arefound, for example, in U.S. Pat. Nos. 7,393,992, 7,482,500, 7,226,527,4,310,388, and 7,132,038, among others.

The butadiene recovery processes typically use 3- or 4-column extractivedistillation systems to separate a mixed C4 stream into three productfractions, including a lights/butane/butenes stream (Raffinate-1product), a crude butadiene product, which may be sent to a conventionaldistillation system for further purification, and a concentrated C4acetylenes stream, which may be sent to a selective hydrogenation unitor recycled to a cracker. The columns used may include a main washcolumn, a rectifier column, which is typically physically builtseparately from the wash column due to height limitations, and anafterwash column, which may be combined with the rectifier column in adivided wall column design.

To increase butadiene recovery from such processes, a conventionalstand-alone pre-fractionator may be used to increase the butadieneconcentration of the dilute feed by conventional distillation.Disadvantages of using conventional distillation pre-fractionationinclude the technical difficulty and cost associated with separating thebutanes/butenes and butadiene, which have relatively low volatility.

Another proposed method to increase butadiene recovery has been toincorporate a stand-alone scrubber/stripper system to process the feedgas, concentrating the butadiene in the feed gas by removing a portionof the butanes/butenes. Disadvantages of using a scrubber/stripper toprocess the feed stream include equipment costs.

In addition, butadiene extraction units may be partially debottleneckedor expanded by replacing the existing trays (valve or sieve) with randompacking (for example, IMTP® High Performance Random Packing availablefrom Koch-Glitsch LP, Wichita, Kans.), or replacing the existing packingwith higher efficiency packing (for example, Raschig Super-Ringsavailable from Raschig GmbH, Ludwigshafen) in all 3 (or 4) columns inthe extractive distillation area. Disadvantages of high efficiencypacking include its inability to increase capacity past a certain point.For example, replacing trays with IMTP packing generally will allow a 25to 40% increase in capacity, and replacing IMTP packing withhigh-capacity packing will generally allow an additional 10 to 15%increase in capacity. Also, the conventional distillation area must alsobe further debottlenecked or expanded to a corresponding degree.

SUMMARY

Embodiments disclosed herein provide improved processes for preparingbutadiene from dilute streams of mixed C₄ hydrocarbons. Morespecifically, embodiments disclosed herein provide for the recovery of acrude butadiene stream from a mixed C4 hydrocarbon stream using abutadiene pre-absorber column, which may be integrated with anextractive distillation column, allowing for the efficient recovery ofbutadiene.

In one aspect, embodiments disclosed herein relate to a process forrecovering butadiene from a C₄ fraction. The process may include:contacting a mixed C₄ stream comprising butane, butene, and butadiene,with a solvent comprising an organic solvent and water in a butadienepre-absorber column to recover an overheads fraction comprising at leasta portion of the butane, butene, and water, and a first bottoms fractioncomprising the organic solvent, butadiene, and at least a portion of thebutene; and feeding the first bottoms fraction to a butadiene extractionunit to recover a butene fraction, a crude butadiene fraction, and asolvent fraction.

The butadiene extraction unit may comprise, for example, a main washcolumn and a rectifier/afterwash column. The pre-absorber bottomsfraction may then be contacted with additional solvent comprising theorganic solvent and water in the main wash column to recover anoverheads fraction comprising the butene and at least a portion of thewater and a second bottoms fraction comprising the organic solvent andbutadiene. The butadiene from the organic solvent may then be separatedin the rectifier/afterwash column to recover the solvent fraction andthe crude butadiene fraction. The solvent recovered, or a portionthereof, may then be recycled to the pre-absorber column and the mainwash column as the organic solvent.

In other embodiments, the pre-absorber and the main wash column mayshare a common overhead system. For example, the overheads fractioncomprising at least a portion of the butane, butene, and water and theoverheads fraction comprising the butene and at least a portion of thewater may be fed to a common overhead condensation system for condensingat least a portion of the combined overheads stream.

The mixed C4 hydrocarbon stream may be provided by at least one ofcracking, oxidatively dehydrogenating, and non-oxidativelydehydrogenating a C₄ hydrocarbon stream comprising butane in one or moredehydrogenation reactors to produce a product gas stream comprisingbutane, butene, and butadiene. In such a case, a portion of one or bothof the overheads fraction comprising at least a portion of the butane,butene, and water and the overheads fraction comprising the butene andat least a portion of the water may be recycled to the upstreambutadiene production process, such as to the one or more dehydrogenationreactors.

In some embodiments, the butadiene pre-absorber column may be operatedsuch that the concentration of butadiene relative to the total C4hydrocarbons in the first bottoms fraction is at least 40 percent byweight. In various embodiments, the organic solvent comprises N-methylpyrrolidone.

In another aspect, embodiments disclosed herein relate to a process forretrofitting a butadiene extraction system for recovering butadiene froma mixed C4 stream comprising butane, butene, and butadiene, the systemcomprising a main wash column for contacting a gaseous mixed C4 streamwith a solvent or solvent mixture to recover an overheads fractioncomprising butane and butene and a bottoms fraction comprising butadieneand the solvent or solvent mixture. The process for retrofitting mayinclude: installing a butadiene pre-absorber column for contacting thegaseous mixed C4 stream with the solvent or solvent mixture to recoveran overheads fraction comprising butane and butene and a bottomsfraction comprising butadiene, at least a portion of the butene, and thesolvent or solvent mixture; fluidly connecting the butadienepre-absorber column with the main wash column for contacting the bottomsfraction with additional solvent to recover an overheads fractioncomprising the butene and a bottoms fraction comprising the butadiene,the solvent, and the additional solvent; and installing a liquiddistributor in the main wash column to distribute the bottoms fractionfed to the main wash column. In some embodiments, the retrofittingprocess may also include fluidly connecting the butadiene pre-absorberto an existing overhead system of the main wash column.

In another aspect, embodiments disclosed herein relate to a system forrecovering butadiene from a mixed C₄ hydrocarbon fraction. The systemmay include: a butadiene pre-absorber column for contacting a mixed C₄stream comprising butane, butene, and butadiene, with a solventcomprising an organic solvent and water to recover an overheads fractioncomprising at least a portion of the butane, butene, and water, and afirst bottoms fraction comprising the organic solvent, butadiene, and atleast a portion of the butene; and a butadiene extraction unit forseparating the first bottoms fraction to recover a butene fraction, acrude butadiene fraction, and a solvent fraction.

The butadiene extraction unit may also include a main wash column forcontacting the bottoms fraction with additional solvent comprising theorganic solvent and water to recover an overheads fraction comprisingthe butene and at least a portion of the water and a second bottomsfraction comprising the organic solvent and butadiene; and arectifier/afterwash column for separating the butadiene from the organicsolvent to recover the solvent fraction and the crude butadienefraction. In various embodiments, the system may also include: one ormore fluid conduits for recycling at least a portion of the solventfraction to the pre-absorber column and the main wash column as theorganic solvent; one or more overheads condensation systems forcondensing at least a portion of i) the overheads fraction comprising atleast a portion of the butane, butene, and water, ii) the overheadsfraction comprising the butene and at least a portion of the water, oriii) a combined overheads stream comprising an admixture of i) and ii);one or more reactors for at least one of cracking, oxidativelydehydrogenating, and non-oxidatively dehydrogenating a C₄ hydrocarbonstream comprising butane in to produce a product gas stream comprisingbutane, butene, and butadiene, and a fluid conduit for feeding at leasta portion of the product gas stream to the butadiene pre-absorbercolumn; one or more fluid conduits for recycling at least a portion ofone or both of the overheads fraction comprising at least a portion ofthe butane, butene, and water and the overheads fraction comprising thebutene and at least a portion of the water to the one or more reactors;and/or a control system for operating the butadiene pre-absorber columnsuch that the concentration of butadiene relative to the total C4hydrocarbons in the first bottoms fraction is at least 40 percent byweight.

Other aspects and advantages will be apparent from the followingdescription and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified flow diagram of a process for butadiene recoveryaccording to embodiments herein.

FIG. 2 is a simplified flow diagram of a process for butadiene recoveryaccording to embodiments herein.

DETAILED DESCRIPTION

Embodiments disclosed here relate to recovery of butadiene from a mixedhydrocarbon stream. More specifically, embodiments disclosed hereinrelate to recovery of a crude butadiene stream from a mixed C4hydrocarbon stream using a butadiene pre-absorber column that may beintegrated with an extractive distillation column for the efficientrecovery of butadiene. It has been found that the extractivedistillation process for the recovery of butadiene may be greatlyimproved by the integration of a pre-absorber column and wash column,and may result in energy and/or separation efficiency allowing for highprocessing rates and expansion of existing butadiene recovery processes.

Referring now to FIG. 1, a simplified process flow diagram forrecovering butadiene from a C4 fraction according to embodimentsdisclosed herein is illustrated. A selective solvent and a mixed C4stream including butanes (n-butane and/or isobutane), butenes (1-butene,2-butene, and/or isobutene), and butadienes (1,2-butadiene and/or1,3-butadiene) may be fed via flow lines 2 and 4, respectively, to abutadiene pre-absorber column 6. Pre-absorber column 6, in someembodiments, may be a single-column absorber, reboiled and refluxed (notillustrated). In the pre-absorber column 6, the mixed C4 stream iscontacted over appropriate internals with the selective solvent,resulting in at least a portion of the butadiene being absorbed in theselective solvent. A portion of the butenes and/or butanes may also beabsorbed by the selective solvent. In some embodiments, the extractivedistillation in the pre-absorber column 6 may partially or fullysaturate the selective solvent with butadiene.

The extractive distillation of the mixed C₄ stream in the pre-absorbercolumn 6 may be operated in such a way that the components of the mixedC₄ stream for which the selective solvent has a lower affinity than forbutadiene, in particular the butanes and the butenes, remain essentiallyin the gas phase, while butadiene and further hydrocarbons for which theselective solvent has a higher affinity than for butadiene are virtuallycompletely absorbed by the selective solvent. An overhead fractionincluding the non-absorbed portion of the butanes and butenes may berecovered from pre-absorber column 6 via flow line 10 (Raffinate-1A).

The selective solvent and extracted butadiene may be recovered as abottoms fraction from the pre-absorber column 6 via flow line 8. Thebottoms fraction, including the absorbed butadiene, may then be fed viaflow line 8 to a butadiene extraction unit 12 for contact withadditional selective solvent fed via flow line 14 over appropriateinternals to further separate the butadiene from the butane and butenesand for separation of the butadiene from the selective solvent. Forexample, butadiene extraction unit 12 may include a main wash column, arectifier, and an afterwash column (not illustrated). Otherconfigurations for butadiene extraction units may also be used. Threeproduct fractions may be recovered from the butadiene extraction unit12, including a butane/butene fraction 16 (Raffinate-1B), a selectivesolvent fraction 18, and a crude butadiene fraction 20. At least aportion of the selective solvent fraction 18 may be recycled to the washcolumn in butadiene extraction unit 12 and/or the pre-absorber column 6.

In some embodiments, the Raffinate-1A and 1B fractions (butane/butenefraction 16 and overhead fraction 10) may be combined and recycled to acracking process or dehydration process for production of additionalbutadiene. The combined Raffinate-1 fractions may be a gaseous streamincluding an enhanced amount of n-butane and 2-butene relative to the C4feed. For example, the combined Raffinate-1 stream including butane andbutene may contain from 50 to 100 volume % n-butane, from 0 to 50 volume%1-butene and 2-butene, and from 0 to 3 volume % other constituents suchas isobutane, isobutene, propane, propylene and C₅₊ hydrocarbons.

In some embodiments, the crude butadiene fraction 20, which may containgreater than 80%, greater than 90%, or greater than 95% by weightbutadiene, with the balance being impurities, may be fractionated toresult in a “pure” butadiene stream, which may contain greater than 99%,greater than 99.5%, or greater than 99.7% butadiene, with the balancebeing impurities.

In the process as illustrated in FIG. 1, the absorbed 1,3-butadiene isconcentrated relative to the absorbed butanes and butenes in the bottomsfraction 8. The absorbed C4s are then fed to a main wash columnpre-absorbed in the selective solvent, thus making the requiredseparations in the main wash column and rectifier less difficult, due tothe higher 1,3-butadiene concentration, and more efficient, as the feedis already saturated. The balance of the butanes and butenes are thenremoved in the main wash column as a distillate product. The integrationof a pre-absorber with the main wash column reduces separationdifficulties, providing for energy efficiency, solvent usageefficiencies, and improved separation efficiency, allowing for increasedoverall throughput for a given main wash column design (with vs. withouta pre-absorber).

The C4 fraction to be used as starting mixture in the present processesis a mixture of hydrocarbons having predominantly four carbon atoms permolecule. C4 fractions are obtained, for example, in the preparation ofethylene and/or propylene by thermal or catalytic cracking of apetroleum fraction, such as liquefied petroleum gas, light naphtha orgas oil. C4 fractions may also be obtained by the catalyticdehydrogenation (oxidative and/or non-oxidative dehydrogenation) ofn-butane and/or n-butene. The resulting C4 fractions generally includebutanes, n-butene, isobutene, 1,3-butadiene and small amounts of C3 andC5 hydrocarbons, as well as butynes, in particular 1-butyne(ethylacetylene) and butenyne (vinylacetylene). The 1,3-butadienecontent is generally from 5 to 80% by weight. For example, a cracker ora CATADIENE unit may contain 15 to 17% butadiene, by weight. Other mixedC4 feed streams may contain greater or lesser amounts of butadiene. Whenpresent in the mixed feed stream, vinylacetylene may be selectivelyhydrogenated to the desired 1,3-butadiene product prior to feed of themixed C4 stream to the pre-absorber.

Selective solvents may include butyrolactone, nitriles such asacetonitrile, propionitrile, methoxypropionitrile, ketones such asacetone, furfural, N-alkyl-substituted lower aliphatic amides such asdimethylformamide, diethylformamide, dimethylacetamide,diethylacetamide, N-formylmorpholine, N-alkyl-substituted cyclic amides(lactams) such as N-alkylpyrrolidones, especially N-methylpyrrolidone(NMP). In some embodiments, alkyl-substituted lower aliphatic amides orN-alkyl-substituted cyclic amides, dimethylformamide, acetonitrile,furfural or NMP are used.

In some embodiments, it is also possible to use mixtures of theseextractants with one another, for example of NMP and acetonitrile,mixtures of these extractants with cosolvents and/or tert-butyl ethers,e.g. methyl tert-butyl ether, ethyl tert-butyl ether, propyl tert-butylether, n- or isobutyl tert-butyl ether. In other embodiments, NMP may bein aqueous solution, with from 0 to about 20 weight % water, or withfrom 7 to 10 weight % water, or with 8 to 8.5 weight % water in otherembodiments.

Referring now to FIG. 2, a simplified process flow diagram of abutadiene pre-absorber according to embodiments disclosed herein isillustrated. A mixed C₄ stream 32 may be fed to a vaporization system34, which may include a vaporization drum 35 and one or more heatexchangers (feed vaporizers) 36, 38 to vaporize the mixed C4 feed. Anintermittent blow down may be taken from vaporization drum 35 to removeheavies that would otherwise build up in the drum. The vaporized C₄s arerecovered via flow line 42 and fed to the bottom of the butadienepre-absorber 48, where they are contacted with a portion of the cold,lean solvent fed via flow line 46. Several wash trays 50 are provided atthe top of butadiene pre-absorber 48 to ensure that solvent does notcontaminate raffinate 10, as well as to minimize solvent losses.

The pre-absorber overheads 47 has an enhanced concentration of butanesand butenes, along with some water; trace concentrations of1,3-butadiene may also be present. The pre-absorber overheads 47 iscondensed in condenser 54, which may be a total or partial condenser,and drains into the accumulator 56. Water 28 is decanted and removed inthe boot of accumulator 56.

A portion of mixed C₄s and solvent is removed from the column by way ofstream 22 and fed to a reboiler 52. Reboiler 52 provides additionalvapor traffic in butadiene pre-absorber 48, which enhances hydrocarbonseparation (i.e., extractive distillation). In some embodiments,reboiler 52 may be used to control the concentration of 1,3-butadienerelative to C₄s at about 40 weight % (for example, in the case of dilutefeed), or higher (for example, in the case of debottleneck/expansions),where the control may be provided via a DCS control system, for example.Reboiler 52, for example, may be a total, once-through reboiler that hasa small percent vaporization and small temperature rise in the case ofdilute feed. In the case of debottleneck/expansions, where high1,3-butadiene concentrations are achieved, a suppressed vaporizationreboiler design option, similar to a rectifier reboiler design, isavailable.

A portion of the condensed overheads, the hydrocarbons recovered in drum56, are refluxed via stream 30 to the wash trays of butadienepre-absorber 48. The reflux of hydrocarbons provided via stream 30washes the butadiene pre-absorber 48 overheads, limiting uptake ofsolvent, and also provides a measure of control for the hydrocarboncomposition profile in butadiene pre-absorber 48. The remaining portionof the condensed overheads, primarily butanes and butenes, being lesssoluble in the solvent than 1,3-butadiene, are removed from drum 56 asraffinate 10.

The absorbed 1,3-butadiene is thus concentrated in the pre-absorberbottoms 8 relative to the feed concentration. The C₄s may then be fed toa butadiene extraction system (not illustrated), which may include amain wash column, a rectifier, and an afterwash column, as they arealready optimally “preabsorbed” in liquid solvent. The extractivedistillation that may occur in the main wash column of the butadieneextraction unit is now more efficient and less costly due to a higher1,3-butadiene concentration, and because the pre-absorber bottoms 8 isalready saturated. In a wash column, the balance of the butanes andbutenes may be removed as a distillate product.

The main wash column may have, for example, from 5 to 15, or from 8 to10, theoretical plates, and a backwash zone having, for example, 4theoretical plates. The backwash zone serves to recover the butadienepresent in the gas phase by means of liquid hydrocarbon reflux, forwhich the top fraction is condensed beforehand. The internals providedare structured packing, trays, or random packing. The pressure at thetop of the column may be, for example, 1 to 2 bara. The temperature inthe bottom of the column may be, for example, from 130 to 150° C.

The extraction solution (selective solvent+absorbed butadiene and otherhydrocarbons) from the main wash column may be transferred to adesorption zone, where the butadiene may be desorbed from the selectivesolvent. The desorption zone may have a reduced pressure and/or elevatedtemperature compared to the pre-absorber and/or main wash column, forexample. The work-up of the selective solvent laden with butadiene (andfurther hydrocarbons for which the selective solvent has a higheraffinity than for butadiene) recovered from the main wash column may becarried out by fractional desorption, with the hydrocarbons absorbed inthe selective solvent being desorbed in the reverse order of theiraffinity for the selective solvent. In some embodiments, thepre-absorber may be integrated with an existing butadiene extractionunit, such as described in U.S. Pat. No. 7,482,500, for example.

As noted above, existing butadiene extraction systems may be retrofittedto include a pre-absorber, such as that illustrated in FIG. 2. In someembodiments, the butadiene pre-absorber shown in FIG. 2 may beintegrated with a pre-existing butadiene extraction unit. In someembodiments, the wash column, rectifier and degasser in a pre-existingbutadiene extraction unit obtaining feed from butadiene pre-absorber 48may require a liquid feed distributor and adjustments to the locationsof their feed due to the C₄s in the pre-absorber bottoms 20 beingpre-absorbed in the liquid solvent. For example, a process forretrofitting a butadiene extraction system for recovering butadiene froma mixed C4 stream may include installing a butadiene pre-absorber columnfor contacting the gaseous mixed C4 stream with the solvent or solventmixture to recover an overheads fraction comprising butane and buteneand a bottoms fraction comprising butadiene, at least a portion of thebutene, and the solvent or solvent mixture; fluidly connecting thebutadiene pre-absorber column with the main wash column for contactingthe bottoms fraction with additional solvent to recover an overheadsfraction comprising the butene and a bottoms fraction comprising thebutadiene, the solvent, and the additional solvent; and installing aliquid distributor in the main wash column to distribute the bottomsfraction fed to the main wash column. In other embodiments, retrofittingan existing unit may also include fluidly connecting the butadienepre-absorber to an existing overhead system of the main wash column ofthe existing butadiene extraction system.

Retrofitting an existing process as described above, to include apre-absorber according to embodiments disclosed herein, may allow for agreater than 40% expansion of capacity, such as up to 50%, 60%, 75%,90%, 100% or even a greater than 100% increase in capacity over existingplant capacity. Without the butadiene pre-absorber process, plantdebottlenecks or expansions (without additional trains) of pre-existingbutadiene extraction processes, such as those incorporating aconventional pre-fractionator or a scrubber/stripper system couldtypically achieve only a 40% expansion at most.

Further with regard to retrofitting an existing butadiene extractionprocess, in some embodiments, the existing main washer may be used asthe pre-absorber together with a new larger main washer, to achieve aneven greater increase in capacity.

EXAMPLE

In Table 1, a process using a conventional butadiene extraction unit iscompared to the same butadiene extraction unit having a butadienepre-absorber similar to that shown in FIG. 2. In this Example, theoverhead system of the butadiene pre-absorber is integrated with theoverhead system of the wash column.

TABLE 1 Conventional Butadiene With Variable\Case Extraction UnitPre-Absorber Solvent to Pre-Absorber N/A 67,835 kg/h Solvent to MainWash 170,150 kg/h 66,700 kg/h Solvent to Aftenvasher 43,350 kg/h 43,035kg/h Total Solvent Flow 213,500 kg/h 177,570 kg/h Pre-Absorber ReboilerN/A 0.53 mm kcal/h Duty Rectifier Reboiler Duty 6.4 mm kcal/h 5.50 mmkcal/h Degasser Reboiler Duty 6.2 mm kcal/h 5.25 mm kcal/h TotalStripping Duty 12.7 mm kcal/h 11.28 mm kcal/h

As shown in Table 1, it is anticipated that the energy efficiency with apre-absorber according to embodiments disclosed herein may be muchgreater than any previous design, such as a conventionalpre-fractionator or a scrubber/stripper system, due in part to thevaporizers and reboilers incorporated in the butadiene pre-absorberprocess. Solvent usage is also significantly decreased for an equivalent1,3-butadiene production rate.

The pre-absorbers according to embodiments disclosed herein may becontrolled using only one hard specification, 1,3-butadiene content inthe distillate, with no hard spec for the butadiene pre-absorberbottoms. This allows for an easier separation control scheme than whenhard specifications are used for both the overheads and bottoms.

Advantageously, the use of pre-absorbers according to embodimentsdisclosed herein may provide for the processing of dilute mixed C₄streams with high efficiency. For example, at equivalent butadienerates, use of a pre-absorber according to embodiments disclosed hereinmay provide for 16.8% lower overall solvent rates and 11% lessutilities, such as shown in Table 1 above, over conventional butadieneextraction systems. Greater efficiencies may also be realized, dependingupon plant capacity and design. Such benefits may be realized usingminimal plot area.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed:
 1. A process for recovering butadiene from a C₄fraction, comprising: contacting a mixed C₄ stream comprising butane,butene, and butadiene, with a solvent comprising an organic solvent andwater in a butadiene pre-absorber column to recover an overheadsfraction comprising at least a portion of the butane, butene, and water,and a first bottoms fraction comprising the organic solvent, butadiene,and at least a portion of the butene; feeding the first bottoms fractionto a butadiene extraction unit to recover a butene fraction, a crudebutadiene fraction, and a solvent fraction, wherein the butadieneextraction unit comprises a main wash column and a rectifier/afterwashcolumn; and contacting the first bottoms fraction with additionalsolvent comprising the organic solvent and water in the main wash columnto recover an overheads fraction comprising the butene and at least aportion of the water and a second bottoms fraction comprising theorganic solvent and butadiene.
 2. The process of claim 1, furthercomprising: separating the butadiene from the organic solvent in therectifier/afterwash column to recover the solvent fraction and the crudebutadiene fraction.
 3. The process of claim 2, further comprisingrecycling at least a portion of the solvent fraction to the pre-absorbercolumn and the main wash column as the organic solvent.
 4. The processof claim 2, further comprising: feeding the overheads fractioncomprising at least a portion of the butane, butene, and water and theoverheads fraction comprising the butene and at least a portion of thewater to a common overhead condensation system for condensing at least aportion of the combined overheads stream.
 5. The process of claim 1,further comprising: at least one of cracking, oxidativelydehydrogenating, and non-oxidatively dehydrogenating a C₄ hydrocarbonstream comprising butane in one or more dehydrogenation reactors toproduce a product gas stream comprising butane, butene, and butadiene;feeding at least a portion of the product gas stream for the contactingin the butadiene pre-absorber column.
 6. The process of claim 5, furthercomprising recycling at least a portion of one or both of the overheadsfraction comprising at least a portion of the butane, butene, and waterand the overheads fraction comprising the butene and at least a portionof the water to the one or more dehydrogenation reactors.
 7. The processof claim 1, further comprising: operating the butadiene pre-absorbercolumn such that the concentration of butadiene relative to the total C4hydrocarbons in the first bottoms fraction is at least 40 percent byweight.
 8. The process of claim 2, further comprising separating waterfrom at least one of the overheads fraction comprising at least aportion of the butane, butene, and water and the overheads fractioncomprising the butene and at least a portion of the water.
 9. Theprocess of claim 4, further comprising separating water from thecombined overheads stream.
 10. The process of claim 2, wherein theorganic solvent comprises N-methyl pyrrolidone.